unit 6 chemical reactions. writing chemical equations all chemical reactions have two parts:...
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Unit 6Chemical Reactions
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Writing Chemical Equations• All chemical reactions have two parts:• ReactantsReactants - the substances you start with (on
left side of arrow)• Products Products - the substances you end up with (on
right side of arrow)• The reactants turn into the products.
Reactants Products
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In a chemical reaction…
• The way atoms are joined is changed.• Atoms aren’t created or destroyed; they just
combine together in new ways.– Law of Conservation of Mass
• Can be described using sentences, word equations, or skeleton equations (symbols):
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#1. In a sentence every item is a word Copper reacts with chlorine to form
copper (II) chloride.#2. In a word equation symbols used w/
element or compound namesCopper + chlorine copper (II) chloride
#3. In a skeleton equation formulas and symbols are used to describe a reaction
Cu + Cl2 CuCl2– These are NOT balanced (# of atoms on each side is
not necessarily equal)
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Symbols used in Equations• The arrow () separates the reactants from the
products; means “reacts” or “yields”
• The plus sign (+) = between products or between reactants; means “and”
• Subscripts are used to describe the number of atoms in a FORMULA.
• Coefficients are used to describe the number of molecules in the REACTION. They are the only things changed when balancing a reaction.
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Subscripts vs. Coefficients
3 CuCl2 + 2 Al 2 AlCl3 + 3 Cu
On the reactant side of the equation:How many copper (II) chloride molecules react?
How many total chlorine atoms are there?
How many aluminum atoms are there?
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States of Matter• Solid--(s) after the formula
– Precipitate -- a solid formed in a reaction
• Gas--(g) after the formula
• Liquid—(l) after the formula
• Aqueous-- (aq) after the formula - dissolved in water.
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Other Symbols used in Equations
• indicates a reversible reaction (More later)
• show that heat is supplied to the reaction
• is used to indicate a catalyst used or supplied, in this case, platinum.
heat ,
Pt
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Remember HON17 !!!
Must Memorize Elements THAT EXIST AS DIATOMIC
MOLECULES
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Write a skeleton equation for:Solid iron (III) sulfide reacts with gaseous hydrogen
chloride to form solid iron (III) chloride and
hydrogen sulfide gas.
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Types of Reactions• There are probably millions of
reactions.• We can’t remember them all, but luckily
they will fall into several categories.• We will learn the 5 major types.
– We will then be able to predict the products of a reaction when given just the reactants!
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Types of Equations
Synthesis (or Combination)Equation in Symbols: A + B ABSample Equation:
2Cu (s) + O2 (g) 2 CuO (s)
2 or more elements combine to form only 1 product
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Types of Equations
DecompositionEquation in Symbols: AB A + BSample Equation:
2 NH3 (g) N2 (g) + 3 H2 (g)
1 reactant breaks down into more than 1 product
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Types of EquationsSingle ReplacementEquation in Symbols: A + BC AB + CSample Equation:
Mg (s) + CuCl2 (aq) Cu (s) + MgCl2 (aq)
• Metal replaces metal; Nonmetal replaces nonmetal– Positive ion replaces positive ion; negative ion
replaces negative ion!
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Types of Equations
Double Replacement – Equation in Symbols: AX + BY BX + AY
Sample Equation: 2AgNO3(aq) + CuCl2 (aq) Cu(NO3)2 (aq) + 2AgCl (s)
2 ionic compounds switching “partners”
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Types of Equations
CombustionEquation in Symbols:
CxHy + O2 CO2 + H2O
Sample Equation: CH4(g) + O2 (g) CO2 (g) + H2O (l)
Hydrocarbons react to form CO2 and H2O
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Examples: Identify the Type of Rxn.1. 2 AgNO3 + Ni Ni(NO3)2 + 2 Ag
2. BaCO3 BaO + CO2
3. 4 Cr + 3 O2 2 Cr2O3
4. Ca + 2 HCl CaCl2 + H2
5. Cu(OH)2 + 2 HC2H3O2 Cu(C2H3O2)2 + 2 H2O
6. C8H18 + O2 CO2 + H2O
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Balancing Chemical Equations• Atoms can’t be created or destroyed in
an ordinary reaction:– All the atoms we start with we must end
up with (meaning: balanced!)• A balanced equation has the same
number of atoms of each element on both sides of the equation.
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Balancing EquationsChemical “bookkeeping” of atoms
involved in the reaction:
H2(g) + O2(g) H2O(g)
H – 2 O – 2Reactants
H – 2 O – 1Products
COEFFICIENTS must be added so reactant atoms EQUAL product atoms!
Note the imbalance in oxygen atoms
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Rules for Balancing Chemical Equations1) Count the number of atoms of each type
of element appearing on both sides
2) Balance the elements one at a time by adding coefficients (the numbers in front) where you need more - save balancing the H and O until LAST!
(HINT: save O until the very last)
3) Double-Check to make sure it is balanced.
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• Never change a subscript to balance an equation (You can only change coefficients)– If you change the subscript (formula) you are
describing a different chemical.– H2O is a different compound than H2O2
• Never put a coefficient in the middle of a formula; they must go only in the front
2NaCl is okay, but Na2Cl is not.
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Hints & Tips for Balancing Equations• Take one element at a time, working from left to right
• Save H for next to last and O for last.
• (Shortcut) polyatomic ions that appear on both sides of the equation should be balanced as independent units
• IF EVERYTHING BALANCES EXCEPT FOR O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is a diatomic element)
– Same goes for HON17
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Remember HON17 !!!
Must Memorize Elements THAT EXIST AS DIATOMIC
MOLECULES
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Balancing Equations Practice
Balance the following chemical equations using the appropriate coefficients:
____ Al(s) + _____ Br2 (l) _____ Al2Br6 (s)
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Balancing Equations Practice
Balance the following chemical equations using the appropriate coefficients:
____ Na3PO4 + ____ Fe2O3 ____ Na2O + ____ FePO4
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Predicting Products - Single Replacement Rxns.
• One element replaces another• Reactants must be 1 element and 1 ionic
compound.• Products will be a different element and a
different compound.
• Na + KCl K + NaCl
• F2 + LiCl LiF + Cl2
(Cations switched)
(Anions switched)
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Predicting Products - Single Replacement Rxns.
• Metals will replace other metals (and they can also replace hydrogen)
• K + AlN • Zn + HCl • Think of water as: HOH
– Metals replace the first H, and then combines with the hydroxide (OH).
• Na + HOH
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Single Replacement Rxns. Practice:
• Fe + CuSO4
• Pb + KCl
• Al + HCl
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Predicting Products - Double Replacement Rxns.
• Two things replace each other.– Reactants must be two ionic compounds, in
aqueous solution
• NaOH + FeCl3 – The positive ions change place.
NaOH + FeCl3 Na+1 Cl-1 + Fe+3 OH-1
NaOH + FeCl3 Fe(OH)3 + NaCl
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Complete and balance:__CaCl2 + __NaOH
__CuCl2 + __K2S
__KOH + __Fe(NO3)3
__(NH4)2SO4 + __BaF2
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Predicting Products – Combustion Rxns.• Combustion means “add oxygen”• Normally, a compound composed of
only C, H, (and maybe O) is reacted with oxygen (O2) – usually called “burning”
• In a combustion reaction, the products will ALWAYS be CO2 and H2O.
C3H8 + O2 CO2 + H2O
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Mixed Practice Examples:• H2 + O2
• H2O
• Zn + H2SO4
• HgO
• KBr + Cl2
• AgNO3 + NaCl
• Mg(OH)2 + H2SO3